## **Core Concept**
Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. It is associated with various physical and developmental delays, intellectual disability, and an increased risk of certain medical conditions.

## **Why the Correct Answer is Right**
The correct answer is related to the characteristics and complications commonly associated with Down syndrome. Individuals with Down syndrome are at an increased risk for certain congenital heart defects, gastrointestinal anomalies, hematological disorders, and skeletal issues.

## **Why Each Wrong Option is Incorrect**
- **Option D:** Normal intelligence is not a characteristic typically associated with Down syndrome. Individuals with Down syndrome usually have some degree of intellectual disability, ranging from mild to severe.
- **Option a (VSD), b (Duodenal atresia), c (Leukaemia), and e (Delayed skeletal maturation):** These are all conditions known to be associated with Down syndrome.
- **VSD (Ventricular Septal Defect)** is a common congenital heart defect in Down syndrome.
- **Duodenal atresia** is a gastrointestinal anomaly that can occur.
- **Leukaemia**, particularly acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML), has an increased incidence.
- **Delayed skeletal maturation** is also commonly observed.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for Down syndrome is the association with an increased risk of **leukemia**, particularly **acute lymphoblastic leukemia (ALL)** and **acute myeloid leukemia (AML)**. Another important association is with **congenital heart defects**, such as **atrial septal defects (ASD)** and **ventricular septal defects (VSD)**, which are among the most common cardiac anomalies.

## **Correct Answer:** D. Normal intelligence